The invention relates to fiber optic telecommunication systems and more specifically to chromatic dispersion compensation in such systems.
The tendency of a pulse of light propagating through an optical fiber to broaden is a result of the fact that different wavelengths of light pass through the fiber at different speeds. This speed differential which causes the pulse to broaden is termed chromatic dispersion. Chromatic dispersion presents a problem in modem optical communication systems because the tendency of light pulses to broaden as they propagate down the fiber causes the closely spaced light pulses to overlap in time. This overlap can have an undesirable effect since it restricts how closely spaced the pulses can be. This in turn limits the data bandwidth of the optical fiber.
There are many characteristics of dispersion. First order dispersion is the rate of change of index of refraction with respect to wavelength in the fiber. First order dispersion is also referred to as group velocity. Second order dispersion is the rate of change of the first order dispersion with respect to wavelength. Second order dispersion produces the pulse broadening. Third order dispersion is the rate of change of broadening with respect to a change in wavelength. This is often referred to as the dispersion slope.
Several solutions have been proposed to mitigate the effects of dispersion in transmission fibers. One technique involves the use of a compensating optical fiber having an appropriate, length and which has a dispersion that is opposite to the dispersion characteristic of the transmission fiber. The result is dispersion in the transmission fiber is substantially matched and canceled by the total dispersion in the compensating fiber. While this technique offers a solution to the dispersion problem, it may be impractical in actual use because of the attenuation due to the required length of the compensating fiber. In such a case, the total transmission length of the fiber is significantly increased thereby increasing the signal attenuation in the fiber. Furthermore, it may be difficult to find a fiber of the desired length with the required dispersion properties.
It is also difficult to design a fiber having a changing index of refraction across the diameter of the fiber (the fiber index profile) that will compensate simultaneously for the second and third dispersion orders. It is even more difficult to control the material properties of such fibers even in the most accurate fabrication process necessary to produce such fibers. In addition, the process of fabricating the single compensating chromatic dispersion fiber is expensive and generally not practical.
When a pulse of light is transmitted through an optical fiber, the energy follows a number of paths which cross the fiber axis at different angles. A group of paths which cross the axis at the same angle is known as a mode. Sometimes it is necessary to limit or control the number of modes used in a transmission system. The fundamental mode LP01 in which light passes substantially along the fiber axis is often used in high bandwidth transmission systems using optical fibers commonly referred to as single mode fibers.
The dispersion properties of high order modes have been investigated at length. There is a dependence of high order mode dispersion on wavelength and on the properties of the fiber. By properly designing the fiber index profile it is possible to make the dispersion slope be positive, negative or zero. It is also possible to make the magnitude of the dispersion be negative, zero or slightly positive. Using these two properties one can either control or compensate for the dispersion in any transmission fiber.
Systems have been developed to take advantage of higher order modes to compensate for dispersion in a typical optical communication system. In such systems it has been necessary to first convert the lower order fundamental mode of the light to a higher order spatial mode. This is accomplished using longitudinal mode conversion.
Prior art methods for mode conversion are known as longitudinal mode conversion and are based on introducing a periodic perturbation along the fiber axis. The length of each period and the number of periods in these longitudinal converters must be determined accurately according to the wavelength, the strength of the perturbation, and the modes involved. By constructing a longitudinal mode converter it is possible to achieve good efficiency in transferring the energy from one mode to the other in a limited spectral bandwidth. This spectral property has been used in Dense Wavelength Division Multiplexing (DWDM) applications in telecommunications for other applications. Unfortunately, this technique is accompanied by significant energy attenuation and it cannot be used over broad spectral bandwidths.
Another deficiency associated with longitudinal mode converters is related to the fact that after the conversion, only a single mode should be present in the fiber. It can be difficult to discriminate between desired modes and undesired modes having almost the same group velocities because unwanted modes can appear at the output of the converter. As the modes propagate, modal dispersion occurs and the pulse broadens. Generally, longitudinal mode converters introduce significant energy attenuation and noise. Therefore, a trade-off must be made between having broad-spectrum capability and the demand for converting the original mode to a pure, single, high-order mode.
One such longitudinal mode converter is discussed in U.S. Pat. No. 5,802,234. Here, a single mode transmission fiber carries the LP01 to a longitudinal mode converter. Before conversion in this system, however, it is necessary to couple the single mode transmission fiber to a multimode fiber while maintaining the signal in the basic LP01 mode. This coupling is typically difficult to achieve without signal degradation and any misalignment or manufacturing inaccuracies can result in the presence of higher order modes. It is desirable that only the LP01 mode propagate initially in the multimode fiber in order to avoid significant noise that degrades the system performance and typically such coupling results in the propagation of additional modes.
The present invention overcomes the disadvantages of longitudinal mode converters and previous attempts to control dispersion in a fiber optic system.
The present invention relates to an apparatus and method for transforming an optical signal between different spatial modes. The apparatus and method are based on a spatially selective phase change of the optical signal wavefront relative to the initial wavefront. As the phase-adjusted optical signal propagates, the transverse intensity distribution changes to correspond to the new spatial mode.
The present invention features a transverse mode transformer having an optical input and a spatially selective retardation element. The retardation element transforms an optical signal received at the optical input from a first spatial mode to a second spatial mode. The retardation element can be a phase plate, a lens, a mirror, a grating, an electro-optic element, a beam splitter or a reflective element. In one embodiment the second spatial mode is of a higher order than the first spatial mode.
In another aspect, the invention features a method of spatial mode transformation which includes the steps of providing a spatially selective retardation element, receiving an optical signal having a first spatial mode at the retardation element and spatially retarding at least a portion of the optical signal to generate a second spatial mode. In one embodiment, the second spatial mode is a higher order mode.